1. Field of the Invention
This invention relates to the production of solvent-containing NCO prepolymers based on a) certain lacquer polyisocyanates and b) certain polyhydroxy polyacrylates, to the NCO prepolymers obtained by this process and to their use for the production of one-component, moisture-curing coating compositions.
2. Description of the Prior Art
Moisture-curing one-component coating compositions based on NCO prepolymers are known (Houben-Weyl, Methoden der Organischen Chemie, Vol. E 20, Georg Thieme Verlag, Stuttgart/New York, 1978). The NCO prepolymers are generally prepared by the reaction of aromatic or aliphatic polyisocyanates with polyether or polyester polyols or with short-chain alcohols.
NCO prepolymers having a relatively high functionality and prepared from low molecular weight alcohols, such as butanediol and/or trimethylol propane, generally lead to coatings having inadequate elasticity when they are used as sole binder in one-component coating compositions.
NCO prepolymers based on polyether polyols generally have inadequate light stabilty and UV stability for practical requirements.
Although NCO prepolymers based on polyester polyols do not suffer from these disadvantages, the ester bonds are sensitive to hydrolysis which often leads to inadequate moisture stability of the resulting coatings.
Accordingly, an object of the present invention is to provide new light- and hydrolysis-stable prepolymers containing free isocyanate groups which are suitable for the production of storage stable, one-component coating compositions that are not attended by the above-described disadvantages of the prior art.
This object may be achieved by the process according to the invention for the production of isocyanate prepolymers based on certain lacquer polyisocyanates and certain polyhydroxy polyacrylates which are described in detail hereinafter.
Although hydroxy-functional polyacrylates are known starting materials for the production of polyurethane lacquers, the lacquers in question are generally two-component polyurethane lacquers, which are only prepared just before use by mixing a polyisocyanate component with the polyacrylate component serving as polyol component. (See, for example, Glasurit-Handbuch, Lacke und Farben, 11th Edition, pages 66 et seq., Curt R. Vincentz Verlag, Hannover 1984).
Storage stable, light-stable NCO prepolymers based on polyhydroxy polyacrylates suitable as binders for moisture-curing coating compositions have not previously been known. DE-OS 3 501 857 is concerned with NCO prepolymers prepared from aromatic polyisocyanates, in particular polyisocyanates or polyisocyanate mixtures of the diphenyl methane series, and polyhydroxy polyacrylates. These prepolymers are used for the production of molded plastics by the reaction injection molding (RIM) process. The polyisocyanate preparations described in this document are not suitable for the production of coating compositions, because they only contain aromatically bound isocyanate groups and, accordingly, are not light-stable. They also have an unacceptably high content of monomeric diisocyanates for use in coating technology because of physiological reasons.
Japanese Patent Application 71 37 819, filed Feb. 6, 1971, (Chemical Abstracts, 78 (1973), Ref. No. 125 963h) is directed to NCO prepolymers in which polyhydroxy polyacrylates are incorporated. However, these NCO prepolymers are based on aromatic polyisocyanates (TDI) which, for this reason alone, render them unsuitable for the production of light-stable coating compositions. There is no indication in this Japanese publication as to which starting materials would have to be reacted together to obtain storage stable, light-stable NCO prepolymers which satisfy the objectives of the present invention.